The number of available antifungal agents is increasing, as is the incidence of significant yeast infections. The National Committee for Clinical Laboratory Standards recently developed a proposed standard for testing yeast susceptibility. There is increasing clinical interest in the results of in vitro tests to help select agents to treat yeast infections. Yet, how well in vitro results correlate with clinical responsiveness remains to be determined. In this study, we intend (1) to compare commercially available testing procedures, such as the E-test (AB Biodisk, Culver City, CA) and a colorimetric method (Alamar Biosciences, Inc., Westlake, OH) with results obtained by the proposed reference method; (2) determine the clinical relevance of in vitro susceptibility results by correlating in vitro and clinical results as much as possible; and (3) to collect clinical yeast isolates and assess any changes over time of susceptibility results. Our work to date suggests that the E-test results are difficult to interpret accurately and the colorimetric test offers the most promise for a rapid and accurate testing procedure that can be performed in a routine diagnostic laboratory. We are waiting for the manufacturer of the plates to resume routine production so we can continue with their evaluation. It may be several years before enough data have been accumulated for us to make any correlation of in vitro and in vivo results.